So, the question is, why does time dilation and the Lorentz contraction happen? Recall that Maxwell's theory states an electromagnetic wave (light) moves at a constant speed, about 670 million miles per hour, a constant known as c. Now imagine a clock that measures the passage of time by counting the number of round trips made by a photon that bounces between two mirrors six inches (about 15 centimeters) apart. The speed of light is constant, so this is a perfectly accurate clock even if it's not very realistic. When the clock is stationary, it would take the photon about a billionth of a second to make one round trip. As the clock begins to moves, the photon has to move not only up and down but also to the side. For each round trip it has to travel a longer distance which takes a longer time. This means the clock will "tick" less and the passage of time will slow down.

Is this just a funny quirk of a light clock? No, it would be the same for any other clock as well. Suppose you strapped another clock to the top mirror of the light clock and synchronized them. They would tick together at the same rate while at rest. As the light clock began to move at a constant speed they would stay in sync because who's to say they weren't still stationary and everything around them was moving? Suppose you could shrink yourself and take a ride on the light clock. If you were to blindfold yourself, you would not be able to tell that you were moving as long as the ride was smooth and the speed constant. You have every right to claim that you are stationary, even if you take off the blindfold, according to the principle of relativity. Because of this principle, any time you discuss speed or velocity (speed and direction), you must say who or what is doing the observing. You also must signify what the speed is relative to if it is moving at a constant velocity.

The one exception to this is light. According to special relativity, everyone measures the same speed for light no matter what you compare it to. This has severe implications. Think about when a ball is thrown at you. Suppose that it is going twelve miles per hour. If you run away from it at a speed of eight miles per hour, from your point of view the ball is now approaching you at a speed of (twelve minus eight) four miles per hour. If you run toward the ball, it speeds up in relation to you. This is not the case with light. If you run away from it, it is still approaching you at the speed of light from your point of view. This holds true if you run toward it, too. So no matter how fast you run toward light, it will still be retreating relative to you at the speed of light. Because of this, it is impossible to catch up with light and so it's impossible to go the speed of light.

Relativity says that the laws of physics are the same as when you are stationary if you are traveling at a constant velocity. For instance, take a situation where Fred and Bob are an equal distance away from a light bulb on either side. Even if they are in a moving train, the situation would be the same as if they were stationary. Assume that the train is moving to the right, Bob is sitting to the left of the light, and Fred is on the right. When you turn on the light bulb in the train, observers in the train see the light reach both Fred and Bob at the same time. However, if you were outside of the train you would disagree. You would say that the light reaches Bob first because the train is moving to the right. The light has a shorter distance to transverse from where it was first emitted to where Bob is than to where Fred is because Fred is moving away from the light and Bob is moving toward it. So who's right? Both viewpoints are correct, which brings us to another axiom of relativity. When two observers are in relative motion, if one claims two events occurred simultaneously the other will disagree. Again, this effect goes unnoticed in everyday life.